This invention relates generally to the art of measuring physical parameters such as temperature by optical techniques, and, more specifically, to implementations that detect a time rate of decay of luminescence that is proportional to the parameter being measured.
Optical techniques using luminescent sensors to measure temperature have in recent years become accepted for many applications and several commercial products have appeared. Some temperature-dependent characteristic of the luminescent emission is measured and the temperature of the sensor determined from that measurement. The temperature of a surface can be measured by making direct contact between such a small luminescent sensor and the surface, or by painting the luminescent material directly on the surface, and then detecting the temperature-dependent luminescence by some type of remote or contact optical system. Such an optical system can use a length of optical fiber as one optical element. This technique has particular application for measuring the temperature of difficult to contact surfaces such as the surface of a rotating piece of machinery.
Commercially available products often attach the luminescent sensor at an end of a length of optical fiber to form a temperature-sensing probe. The temperature sensing probe is then placed in contact with an object, or within an environment, whose temperature is to be measured. The other end of the optical fiber is then connected to a measuring instrument. Since neither the sensor nor the optical fiber contain electrically conducting materials, temperatures may be measured in environments that are hostile to other measuring systems such as in high voltage fields, intense electromagnetic radiation fields or beams, or environments which contain chemicals which might corrode electrical sensors or their metallic leads. Optical fiber probes may also be made to be implanted in the human body for measuring internal body fluid or tissue temperatures. Since the optical fibers do not conduct electricity, the probes can be used in the processing of flammable or explosive materials where electrically safe sensors are required. Also, since the sensors have very small mass and since the fibers do not conduct heat away from the measurement location, more accurate sensing is possible. Further, such fiberoptic probes may be permanently installed in large, expensive electrical equipment, such as electrical generators and power transformers. The very small size of the sensor and optical fiber cable, as well as its immunity from environmental factors that prevent more conventional temperature sensing techniques from being used, contribute to a wide range of additional applications.
The temperature-dependent characteristic of luminescence that is emerging as preferred for use in commercial systems is the temperature sensitivity of its time rate of decay. Luminescent materials used as temperature sensors, in response to a pulse of radiation that causes them to commence luminescence, exhibit a decay of their luminescence, after termination of the exciting pulse, with a rate which varies with temperature. The most desirable luminescent materials for such use exhibit an exponential decay of luminescent intensity after cessation of the excitation radiation. This then allows the temperature being measured to be correlated with a decay "time constant" of the luminescence, a quantity normally referenced as .tau., which is defined as the time to reach 1/e of the initially-measured value of the decaying luminescence, where "e" is the natural logarithm base 2.71828 . . . . The advantage of monitoring luminescent decay time (or some other parameter related to it) is that the measurements are insensitive to changes in intensity of the signal that can occur over time as photodetectors and excitation sources change their characteristics with aging, or which occur during use such as may be caused by bending the optical fiber, which immediately changes the amount of attenuation of the light being passed through it. Other more gradual changes in fiber transmission can be caused by exposing the fiber to ionizing radiation which then causes increased absorption at certain wavelengths, by exposing the fiber to various liquids which can alter the index of refraction of the cladding layer which in turn alters the transmission of the fiber, or by exposing the fiber to a changing thermal environment which changes the relative indices of refraction of the fiber core and cladding.
There are several analog techniques for measuring a temperature dependent characteristic of the luminescent decay which process the electrical signal output of the photodetector in different ways. One technique is to measure the intensity level at two times during the decay process. Examples of this are described in U.S. Pat. Nos. 4,223,226--Quick et al., (1980), 4,652,143--Wickersheim et al. (1987), and 4,789,992--Wickersheim et al. (1988). Another technique is to excite the luminescent material sensor by driving the excitation source with a sine wave, and then comparing with the excitation signal the relative phase of the resulting luminescent signal. The phase difference is proportional to the temperature-dependent luminescent decay time. This is described in U.S. Pat. Nos. Re. 31,832--Samulski (1985) and 4,437,772--Samulski (1984), and in the following published papers: Augousti et al., "A Laser-Pumped Temperature Sensor Using the Fluorescent Decay Time of Alexandrite", Journal of Lightwave Technology, Volume LT-5, No. 6, June 1987, pages 759-762; Grattan et al., "Ruby Decay-Time Fluorescence Thermometer in a Fiber-Optic Configuration", Rev. Sci. Instrum., Volume 59, No. 8, August 1988, pages 1328-1335; and Augousti et al., "Visible-LED Pumped Fiber-Optic Temperature Sensor", IEEE Transactions on Instrumentation and Measurement, Volume 37, No. 3, September 1988, pages 470-472. A similar concept for measuring free oxygen concentrations by means of the effect of the oxygen on the decay time of luminescent material exposed thereto is described in U.S. Pat. No. 4,716,363--Dukes et al (1987).
Another way to measure the decay time is to integrate the photodetector output for a time, thereby calculating the area under the decaying intensity curve. A ratio of integrals can then be employed to provide a self-referenced (intensity independent) measurement. A proposal for doing this was made by Sholes et al., "Fluorescent Decay Thermometer with Biological Applications", Rev. Sci. Instrum., Volume 51, No. 7, July 1980, pages 882-884. Another integration and ratioing technique has been described in U.S. Pat. No. 4,776,827--Greaves (1988).
More recent signal processing techniques include a closed loop, feedback circuit between the photodetector and excitation source with a characteristic of the feedback signal being proportional to temperature. In one such system, a voltage-controlled oscillator (VCO) drives an excitation light source at a frequency determined by the decay time being measured. This is described by Bosselmann et al., "Fiber-Optic Temperature Sensor Using Fluorescence Decay Time", Proceedings of the 2nd International Conference on Optical Fiber Sensors, pages 151-154, in British Patent No. 2,113,837--Bosselmann (1986), and in European patent application publication No. 174,506--Franke et al. (1986). Another closed-loop system described in U.S. Pat. No. 4,816,687--Fehrenbach et al. (1989) uses an integrator in the feedback loop that provides a quantity proportional to decay time and hence temperature.
The luminescent sensors in such systems are typically excited by a flash lamp, a laser or a light-emitting diode (LED). A flash lamp provides a short burst of excitation energy in response to an electrical pulse. LEDs may be pulsed much more rapidly, and their output can be much more precisely controlled in terms of consistency of light intensity. LEDs are also less expensive, easier to work with, have a longer life, require less power to operate, and generate less heat that needs to be dissipated. In the closed-loop systems described above, an LED is utilized to provide a periodically fluctuating light intensity function to excite the luminescent sensor.
Since the most efficient LEDs have a near infrared or red light output, their use does place some constraints on the composition of the luminescent material that is utilized in the sensor. The chosen luminescent material must absorb in the wavelength band emitted by the LED, with the desired resultant fluorescent emission occurring at slightly longer wavelengths that are still short enough to be detectable by fast, sensitive photodetectors such as photomultipliers or photodiodes.
The analog techniques and systems described above may give results that are subject to some non-thermal changes over time, but this is generally accepted as inevitable and compensated for by providing for occasional system recalibration. Causes of drift and changes over time most frequently result from gradual changes in the characteristics of analog electronic components such as those used in amplifiers, comparators and the like.
It is therefore a principal object of the present invention to provide a luminescent sensor-based measurement system that is usable with confidence over a long period of time to provide accurate results with consistency.
In addition to the analog techniques which are being commercialized, digital techniques have been suggested. Examples are found in the following papers: Dowell et al., "Precision Limits of Waveform Recovery and Analysis in a Signal Processing Oscilloscope", Rev. Sci. Instrum., Volume 58, No. 7, July 1987, pages 1245-1250; Lutz et al., "Thermographic Phosphors: An Alternative to Bare Wire Type K Thermocouples at High Temperatures", Industrial Heating, Volume 54, No. 10, Oct. 1987, pages 36-41; and Mannik et al., "The Application of Phosphor Thermometry to Generator Rotor Temperature Monitoring", Electro-Optic Sensing and Measurement, ICALEO '87 Proceedings, Nov. 8-12, 1987, pages 23-27. However, a fully digital approach has not been implemented to-date in a small, economical measurement instrument. This is in part because digital processing chips having the necessary speed and capacity have only recently become available at a reasonable cost.
Therefore, it is another object of the present invention to provide a luminescent sensor-based measuring system that utilizes digital signal processing in a manner that allows the system to be made very small and compact, have low power requirements, and exhibit a high degree of ruggedness.
It is also an object to provide a small and relatively inexpensive module which can be used in portable or OEM instrumentation applications.
It is another object of the present invention to provide a luminescent sensor-based measuring technique that can be implemented with low-cost components and be practical for use in many different specific applications.
It is yet another object of the present invention to provide such a technique that can yield precise measurement results in short periods of time, and, if necessary, can operate reliably even when the luminescent intensity from the sensor is relatively weak because of extended fiber length, a large number of connections, or long-term degradation such as might be caused by LED aging.